Protein kinases are a family of enzymes that catalyze phosphorylation of the hydroxyl group of specific tyrosine, serine, or threonine residues in proteins. Typically, such phosphorylation dramatically perturbs the function of the protein, and thus protein kinases are pivotal in the regulation of a wide variety of cellular processes, including metabolisim, cell proliferation, cell differentiation, and cell survival. Of the many different cellular functions in which the activity of protein kinases is known to be required, some processes represent attractive targets for therapeutic intervention for certain disease states. Two examples are angiogenesis and cell-cycle control, in which protein kinases play a pivotal role; these processes are essential for the growth of solid tumors as well as for other diseases.
Angiogenesis is the mechanism by which new capillaries are formed from existing vessels. When required, the vascular system has the potential to generate new capillary networks in order to maintain the proper functioning of tissues and organs. In the adult, however, angiogenesis is fairly limited, occurring only in the process of wound healing and neovascularization of the endometrium during menstruation. See Merenmies et al., Cell Growth & Differentiation, 8, 3–10 (1997). On the other hand, unwanted angiogenesis is a hallmark of several diseases, such as retinopathies, psoriasis, rheumatoid arthritis, age-related macular degeneration (AMD), and cancer (solid tumors). Folkman, Nature Med., 1, 27–31 (1995). Protein kinases which have been shown to be involved in the angiogenic process include three members of the growth factor receptor tyrosine kinase family: VEGF-R2 (vascular endothelial growth factor receptor 2, also known as KDR (kinase insert domain receptor) and as FLK-1); FGF-R (fibroblast growth factor receptor); and TEK (also known as Tie-2).
VEGF-R2, which is expressed only on endothelial cells, binds the potent angiogenic growth factor VEGF and mediates the subsequent signal transduction through activation of its intracellular kinase activity. Thus, it is expected that direct inhibition of the kinase activity of VEGF-R2 will result in the reduction of angiogenesis even in the presence of exogenous VEGF (see Strawn et al., Cancer Research, 56, 3540–3545 (1996)), as has been shown with mutants of VEGF-R2 which fail to mediate signal transduction. Millauer et al., Cancer Research, 56, 1615–1620 (1996). Furthermore, VEGF-R2 appears to have no function in the adult beyond that of mediating the angiogenic activity of VEGF. Therefore, a selective inhibitor of the kinase activity of VEGF-R2 would be expected to exhibit little toxicity.
Similarly, FGF-R binds the angiogenic growth factors aFGF and bFGF and mediates subsequent intracellular signal transduction. Recently, it has been suggested that growth factors such as bFGF may play a critical role in inducing angiogenesis in solid tumors that have reached a certain size. Yoshiji et al., Cancer Research, 57, 3924–3928 (1997). Unlike VEGF-R2, however, FGF-R is expressed in a number of different cell types throughout the body and may or may not play important roles in other normal physiological processes in the adult. Nonetheless, systemic administration of a small-molecule inhibitor of the kinase activity of FGF-R has been reported to block bFGF-induced angiogenesis in mice without apparent toxicity. Mohammad et al., EMBO Journal, 17, 5996–5904 (1998).
TEK (also known as Tie-2) is another receptor tyrosine kinase expressed only on endothelial cells which has been shown to play a role in angiogenesis. The binding of the factor angiopoietin-1 results in autophosphorylation of the kinase domain of TEK and results in a signal transduction process which appears to mediate the interaction of endothelial cells with peri-endothelial support cells, thereby facilitating the maturation of newly formed blood vessels. The factor angiopoietin-2, on the other hand, appears to antagonize the action of angiopoietin-1 on TEK and disrupts angiogenesis. Maisonpierre et al., Science, 277, 55–60 (1997).
As a result of the above-described developments, it has been proposed to treat angiogenesis by the use of compounds inhibiting the kinase activity of VEGF-R2, FGF-R, and/or TEK. For example, WIPO International Publication No. WO 97/34876 discloses certain cinnoline derivatives that are inhibitors of VEGF-R2, which may be used for the treatment of disease states associated with abnormal angiogenesis and/or increased vascular permeability such as cancer, diabetes, psoriasis, rheumatoid arthritis, Kaposi's sarcoma, haemangioma, acute and chronic nephropathies, atheroma, arterial restinosis, autoimmune diseases, acute inflammation, and ocular diseases with retinal vessel proliferation.
Phosphorylase kinase activates glycogen phosphorylase, thus increasing glycogen breakdown and hepatic glucose release. Hepatic glucose production is disregulated in type 2 diabetes, and is the primary cause of fasting hyperglycemia, which results in many of the secondary complications afflicting these patients. Thus, reduction in glucose release from the liver would lower elevated plasma glucose levels. Inhibitors of phosphorylase kinase should therefore decrease phosphorylase activity and glycogenolysis, thus reducing hyperglycemia in patients.
Another physiological response to VEGF is vascular hyperpermeability, which has been proposed to play a role in the early stages of angiogenesis. In ischemic tissues, such as those occurring in the brain of stroke victims, hypoxia trigger VEGF expression, leading to increased vascular permeability and ultimately edema in the surrounding tissues. In a rat model for stroke, it has been shown by van Bruggen et al., J. Clinical Invest, 104, 1613–20 (1999) that administration of a monoclonal antibody to VEGF reduces the infarct volume. Thus, inhibitors of VEGFR are anticipated to be useful for the treatment of stroke.
In addition to its role in angiogenesis, protein kinases also play a crucial role in cell-cycle control. Uncontrolled cell proliferation is the insignia of cancer. Cell proliferation in response to various stimuli is manifested by a de-regulation of the cell division cycle, the process by which cells multiply and divide. Tumor cells typically have damage to the genes that directly or indirectly regulate progression through the cell division cycle.
Cyclin-dependent kinases (CDKs) are serine-threonine protein kinases that play critical roles in regulating the transitions between different phases of the cell cycle. See, e.g., the articles compiled in Science, 274, 1643–1677 (1996). CDK complexes are formed through association of a regulatory cyclin subunit (e.g., cyclin A, B1, B2, D1, D2, D3, and E) and a catalytic kinase subunit (e.g., cdc2 (CDK1), CDK2, CDK4, CDK5, and CDK6). As the name implies, the CDKs display an absolute dependence on the cyclin subunit in order to phosphorylate their target substrates, and different kinase/cyclin pairs function to regulate progression through specific phases of the cell cycle.
It is CDK4 complexed to the D cyclins that plays a critical part in initiating the cell-division cycle from a resting or quiescent stage to one in which cells become committed to cell division. This progression is subject to a variety of growth regulatory mechanisms, both negative and positive. Aberrations in this control system, particularly those that affect the function of CDK4, have been implicated in the advancement of cells to the highly proliferative state characteristic of malignancies, particularly familial melanomas, esophageal carcinomas, and pancreatic cancers. See, e.g., Kamb, Trends in Genetics, 11, 136–140 (1995); Kamb et al., Science, 264, 436–440 (1994).
Myriad publications describe a variety of chemical compounds useful against a variety of therapeutic targets. For example, WIPO International Publication Nos. WO 99/23077 and WO 99/23076 describe indazole-containing compounds having phosphodiesterase type IV inhibitory activity produced by an indazole-for-catechol bioisostere replacement. U.S. Pat. No. 5,760,028 discloses heterocycles including 3-[1-[3-(imidazolin-2-ylamino)propyl]indazol-5-ylcarbonylamino]-2-(benzyloxycarbonylamino)propionic acid, which are useful as antagonists of the αvβ3 integrin and related cell surface adhesive protein receptors. WIPO International Publication No. WO 98/09961 discloses certain indazole derivatives and their use as inhibitors of phosphodiesterase (PDE) type IV or the production of tumor necrosis factor (TNF) in a mammal. Recent additions to the virtual library of known compounds include those described as being anti-proliferative therapeutic agents that inhibit CDKs. For example, U.S. Pat. No. 5,621,082 to Xiong et al. discloses nucleic acid encoding an inhibitor of CDK6, and European Patent Publication No. 0 666 270 A2 describes peptides and peptide mimetics that act as inhibitors of CDK1 and CDK2. WIPO International Publication No. WO 97/16447 discloses certain analogs of chromones that are inhibitors of cyclin-dependent kinases, in particular of CDK/cyclin complexes such as CDK4/cyclin D1, which may be used for inhibiting excessive or abnormal cell proliferation, and therefore for treating cancer. WIPO International Publication No. WO 99/21845 describes 4-aminothiazole derivatives that are useful as CDK inhibitors.
There is still a need, however, for small-molecule compounds that may be readily synthesized and are effective in inhibiting one or more CDKs or CDK/cyclin complexes. Because CDK4 may serve as a general activator of cell division in most cells, and complexes of CDK4 and D-type cyclins govern the early G1 phase of the cell cycle, there is a need for effective inhibitors of CDK4, and D-type cyclin complexes thereof, for treating one or more types of tumors. Also, the pivotal roles of cyclin E/CDK2 and cyclin B/CDK1 kinases in the G1/S phase and G2/M transitions, respectively, offer additional targets for therapeutic intervention in suppressing deregulated cell-cycle progression in cancer.
Another protein kinase, CHK1, plays an important role as a checkpoint in cell-cycle progression. Checkpoints are control systems that coordinate cell-cycle progression by influencing the formation, activation and subsequent inactivation of the cyclin-dependent kinases. Checkpoints prevent cell-cycle progression at inappropriate times, maintain the metabolic balance of cells while the cell is arrested, and in some instances can induce apoptosis (programmed cell death) when the requirements of the checkpoint have not been met. See, e.g., O'Connor, Cancer Surveys, 29, 151–182 (1997); Nurse, Cell, 91, 865–867 (1997); Hartwell et al., Science, 266, 1821–1828 (1994); Hartwell et al., Science, 246, 629–634 (1989).
One series of checkpoints monitors the integrity of the genome and, upon sensing DNA damage, these “DNA damage checkpoints” block cell-cycle progression in G1 and G2 phases, and slow progression through S phase. O'Connor, Cancer Surveys, 29, 151–182 (1997); Hartwell et al., Science, 266, 1821–1828 (1994). This action enables DNA repair processes to complete their tasks before replication of the genome and subsequent separation of this genetic material into new daughter cells takes place. Importantly, the most commonly mutated gene in human cancer, the p53 tumor suppressor gene, produces a DNA damage checkpoint protein that blocks cell-cycle progression in G1 phase and/or induces apoptosis (programmed cell death) following DNA damage. Hartwell et al., Science, 266, 1821–1828 (1994). The p53 tumor suppressor has also been shown to strengthen the action of a DNA damage checkpoint in G2 phase of the cell cycle. See, e.g., Bunz et al., Science, 28, 1497–1501 (1998); Winters et al., Oncogene, 17, 673–684 (1998); Thompson, Oncogene, 15, 3025–3035 (1997).
Given the pivotal nature of the p53 tumor suppressor pathway in human cancer, therapeutic interventions that exploit vulnerabilities in p53-defective cancer have been actively sought. One emerging vulnerability lies in the operation of the G2 checkpoint in p53 defective cancer cells. Cancer cells, because they lack G1 checkpoint control, are particularly vulnerable to abrogation of the last remaining barrier protecting them from the cancer-killing effects of DNA-damaging agents: the G2 checkpoint. The G2 checkpoint is regulated by a control system that has been conserved from yeast to humans. Important in this conserved system is a kinase, CHK1, which transduces signals from the DNA-damage sensory complex to inhibit activation of the cyclin B/Cdc2 kinase, which promotes mitotic entry. See, e.g., Peng et al., Science, 277, 1501–1505 (1997); Sanchez et al., Science, 277, 1497–1501 (1997). Inactivation of CHK1 has been shown to both abrogate G2 arrest induced by DNA damage inflicted by either anticancer agents or endogenous DNA damage, as well as result in preferential killing of the resulting checkpoint defective cells. See, e.g., Nurse, Cell, 91, 865–867 (1997); Weinert, Science, 277, 1450–1451 (1997); Walworth et al., Nature, 363, 368–371 (1993); and Al-Khodairy et al., Molec. Biol. Cell, 5, 147–160 (1994).
Selective manipulation of checkpoint control in cancer cells could afford broad utilization in cancer chemotherapeutic and radiotherapy regimens and may, in addition, offer a common hallmark of human cancer “genomic instability” to be exploited as the selective basis for the destruction of cancer cells. A number of factors place CHK1 as a pivotal target in DNA-damage checkpoint control. The elucidation of inhibitors of this and functionally related kinases such as Cds1/CHK2, a kinase recently discovered to cooperate with CHK1 in regulating S phase progression (see Zeng et al., Nature, 395, 507–510 (1998); Matsuoka, Science, 282, 1893–1897 (1998)), could provide valuable new therapeutic entities for the treatment of cancer.
Integrin receptor binding to ECM initiates intracellular signals mediated by FAK (Focal Adhesion Kinase) that are involved in cell motility, cellular proliferation, and survival. In human cancers, FAK overexpression is implicated in tumorigenesis and metastatic potential through its role in integrin mediated signaling pathways.
Tyrosine kinases can be of the receptor type (having extracellular, transmembrane and intracellular domains) or the non-receptor type (being wholly intracellular). At least one of the non-receptor protein tyrosine kinases, namely, LCK, is believed to mediate the transduction in T-cells of a signal from the interaction of a cell-surface protein (Cd4) with a cross-linked anti-Cd4 antibody. A more detailed discussion of non-receptor tyrosine kinases is provided in Bolen, Oncogene, 8, 2025–2031 (1993), which is incorporated herein by reference.
In addition to the protein kinases identified above, many other protein kinases have been considered to be therapeutic targets, and numerous publications disclose inhibitors of kinase activity, as reviewed in the following: McMahon et al, Oncologist, 5, 3–10 (2000); Holash et al., Oncogene, 18, 5356–62 (1999); Thomas et al., J. Biol. Chem., 274, 36684–92 (1999); Cohen, Curr. Op. Chem. Biol., 3, 459–65 (1999); Klohs et al., Curr. Op. Chem. Biol., 10, 544–49 (1999); McMahon et al., Current Opinion in Drug Discovery & Development, 1, 131–146 (1998); Strawn et al., Exp. Opin. Invest. Drugs, 7, 553–573 (1998). WIPO International Publication WO 00/18761 discloses certain substituted 3-cyanoquinolines as protein kinase inhibitors.
There is still a need, however, for effective inhibitors of protein kinases. Moreover, as is understood by those skilled in the art, it is desirable for kinase inhibitors to possess both high affinity for the target kinase or kinases as well as high selectivity versus other protein kinases.